The isoflavones (soybean isoflavones: daidzein, genistein, glycitein) contained in soybeans have structures similar to estradiol, and have anti-estrogen actions associated with binding to estrogen receptors (hereinafter referred to as ER) and estrogen-like actions. The ever conducted epidemiological studies and intervention studies of soybean isoflavones suggest that they have preventive effects due to their anti-estrogen actions on breast cancer, prostate cancer and other hormone-dependent cancers and improving effects due to their estrogen-like actions on menopausal disorders, postmenopausal osteoporosis and hyperlipidemia.
Recently, it has been pointed out that the active component of the physiological effects of these soybean isoflavones may be a metabolite of daidzein, i.e., equol. More specifically, it has been reported that equol has an ability to bind to ER (especially to ERβ) greater than soybean isoflavones and that it has a remarkably high transition capability to target organs such as breast and prostate tissues (Non-Patent Documents 1 to 4). Moreover, a case-control study reports that there are significantly less patients who produce equol in the patients of breast cancer and prostate cancer. The effects of soybean isoflavones that improve bone density and lipid metabolism were examined regarding postmenopausal women categorized into two groups: those who produce equol and those who do not. A significant improvement in those who produce equol was observed.
Equol is produced by metabolism of daidzein by enteric bacteria. The abilities to produce equol vary between individuals, and the percentage of Japanese who produce equol is reportedly about 50%. That is, about 50% of Japanese are not able to produce equol (non-equol-producing individuals). Such an individual cannot enjoy any useful physiological benefits based on the action of equol even if they ingest soybeans and processed soybean foods. Therefore, in order to attain useful physiological benefits based on the action of equol in a non-equol-producing individual, ingesting equol itself is thought to be effective.
Non-patent Document 1: Morito K, Hirose T, Kinjo J, Hirakawa T, Okawa M, Nohara T, Ogawa S, Inoue S, Muramatsu M, Masamune Y. Interaction of phytoestrogens with estrogen receptors α and β. Biol Pharm Bull 24(4): 351-356, 2001
Non-patent Document 2: Maubach J, Bracke M E, Heyerick A, Depypere H T, Serreyn R F, Mareel M M, Keukeleire D D. Quantitation of soy-derived phytoestrogens in human breast tissue and biological fluids by high-performance liquid chromatography. J Chromatography B 784: 137-144, 2003
Non-patent Document 3: Morton M S, Chan P S F, Cheng C, Blacklock N, Matos-Ferreira A, Abranches-Monteiro L, Correia R, Lloyd S, Griffiths K. Lignans and isoflavonoids in plasma and prostatic fluid in men: Samples from Portugal, Hong Kong, and the United Kingdom. Prostate 32: 122-128, 1997
Non-patent Document 4: Tammy E H, Paul D M, Paul G F, Robert D, Stephen B, Kenneth J, Ray M, Lorraine G O, Kristiina W, Holly M S, Karen J G. Long-term dietary habits affect soy isoflavone metabolism and accumulation in prostatic fluid in caucasian men. J Nutr 135: 1400-1406, 2005.